The QuaLAT project: mapping QTL for milk fatty acid content in the Italian Brown population and in the Israel Holstein Friesian

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Milk contains a number of micro-components (“functional foods”) having specific health promoting activities. Important among these compounds are the conjugated linoleic acids (CLAs) consistingof a mixture of isomers of C18:2 fatty acid with conjugated doublebonds. The predominant form in milk fat is the cis-9,trans-11 isomer, accounting for 75 to 90% of the total CLA content. Biomedical studies with animal models have demonstrated a variety of preventive health effects from CLA including anticarcinogenic, antiatherogenic, antiobesity, immune system enhancement and antidiabetic benefits. Linolenic acid does not form CLA as an intermediate in rumen biohydrogenation, but it does form vaccenic acid (C18:1,t11) that enters the bloodstream from the rumen, through the abomasum and intestine, and is transported to the mammary gland. Several investigations using direct and indirect approaches established that endogenous synthesis of CLA via ∆9-desaturase action on vaccenic acid (VA, C18:1,t11) is the predominant source of the CLA found in milk fat across a range of diets. Thus, rumen VA production and mammary tissue ∆9-desaturase (D9D) are of key importance in determining the CLA content of milk fat. Mammary tissue D9D activity is measured indirectly as the milk desaturase index (DI), calculated as the ratio of milk CLA to the sum of milk CLA and VA. The study is centered on the Italian Brown Swiss and Israeli Holstein dairy cattle populations. Objectives of the project are: i) to identify environmental and physiological factors (e.g., herd, season, parity and stage of lactation) affecting milk CLA content and DI, and determine appropriate correction factors, if needed; ii) to estimate heritability of milk CLA content and DI in the study populations; iii) to implement a total genome scan for QTL affecting milk CLA VA content and DI in the two study populations. using a selective DNA pooling approach based on milk samples for the initial scan, followed by individual genotyping to confirm suggestive QTL regions. For each of the studied populations,, the research includes determination of CLA, VA and DI for more than 2500 individuals of five large sire half-sib families (for the genome scan) and an additional 400 individuals of 20 small sire half-sib families for the heritability analysis. Milk fat is extracted and transmethylated according to Chouinard et al. (1999). Fatty acid methyl esters are analyzed by GC-FID with a highly polar 100 m SP-2560 column, using GLC-60 supplemented with CLA and VA as a standard. Based on the results obtained to date for the Italian Brown Swiss population, the mean (fatty acid composition over the total fatty acid) ±SD( minimum and maximum) are respectively: VA, 1.97±0.77 (0.63 - 5.40); CLA, 0.63±.0.47 (0.35 - 5.31); DI, 0.25±0.097 (0.063 - 0.64). The means for VA and CLA content found are similar to those reported by other studies (control treatment) and reviewed by Collomb et al (2006). The slightly larger variability here reported, anyhow within the limit of values found in literature, is possibly affected by the wide farming conditions of the individuals sampled.